Why the Latest Windows 11 Update Breaks Local Web Servers and How to Fix It

Microsoft's October Windows 11 update (KB5066835) broke the ability of local web servers to run, preventing developers from accessing web applications on their own machines.

The issue first appeared on Microsoft’s own support forums and quickly spread to Stack Overflow and Server Fault, effectively cutting off Windows’ self‑hosting capability.

Developers reported HTTP/2 protocol errors and failed connections that affected ASP.NET applications built and debugged in Visual Studio.

The vulnerability is present in version 26100.6899 and has been traced to HTTP.sys, the Windows kernel component that handles local HTTP traffic. Uninstalling KB5066835 (or, in some cases, its sibling KB5065789) restores localhost functionality.

A temporary workaround involves manually disabling HTTP/2 in the Windows registry, which works but is considered a blunt solution.

Users who tried reinstalling the patch or rolling back to a newer version experienced mixed results. The problem disappears on a fresh installation of Windows 11 24H2, suggesting the bug stems from a conflict between the update and existing system configurations rather than a universal flaw.

Stack Overflow moderators have locked several related posts, and Server Fault threads are filled with frustrated developers attempting to get their local servers running again.

All of this occurred as Microsoft released the final Windows 10 update, officially ending support for the decade‑old OS and urging users to migrate to Windows 11.

The transition has not been smooth: the Windows 11 Media Creation Tool stopped working the day before, potentially affecting users trying to upgrade, and support for Office 2019 and several server products has also ended.

In the same week, Microsoft’s installer crashed, and the new operating system hindered local web development, highlighting the fragility of its ecosystem despite a multi‑million‑dollar upgrade effort.

After 24 hours of pressure for a comment, a Microsoft spokesperson confirmed the issue, stating that mitigation measures are being actively taken and advising customers to follow the provided guidance.